Method and Device for Transmitting a Signal in a Multi-Antenna System, Signal, and Method for Estimating the Corresponding Transmission Channels

ABSTRACT

A method for transmitting a digital signal via n transmit antennas, wherein n is strictly greater than 2, comprising the steps of combining with a source data vector n vectors to be transmitted respectively by each of the transmit antennas by a coding matrix &lt;I&gt;M&lt;/I&gt; with a yield equal to 1, using reference symbols known to at least one receiver whereby it is able to estimate at least three transmission channels corresponding respectively to each of said transmit antennas. Said coding matrix &lt;I&gt;M&lt;/I&gt; applied a mathematical transformation to the reference symbols prior to the transmission thereof.

CROSS-REFERENCE TO RELATED APPLICATION

This Application is a Section 371 National Stage Application ofInternational Application No. PCT/FR2005/000965, filed Apr. 19, 2005 andpublished as WO 2005/112329 on Nov. 24, 2005, not in English.

FIELD OF THE DISCLOSURE

The field of the disclosure is that of digital communications bywireless. More specifically, the disclosure relates to the transmissionand reception, and especially the estimation of transmission channels ina MIMO (“Multiple Input Multiple Output”) type or MISO (“Multiple InputSingle Output”) type multiple-antenna system, through the transmissionof signals subjected to a space-time and/or space-frequency encoding.

More specifically again, the invention can be applied to multi-antennasystems implementing several transmit antennas, in particular more thantwo transmit antennas. The signals comprise reference symbols, known toat least one receiver and enabling this receiver to estimate thetransmission channels corresponding to each of the transmit antennas.

An example of an application of an embodiment of the invention is in thefield of radio communications, especially for third, fourth andsubsequent generation systems.

An embodiment of the invention can be applied to uplink communications(from a terminal to a base station) as well as to downlinkcommunications (from a base station to a terminal).

BACKGROUND

There are several and known techniques of estimation of transmissionchannels in a multi-antenna system comprising several transmit antennas.

Most of these estimation techniques are limited to the application of aspace-time encoding or space-frequency encoding in OFDM-typemulti-carrier systems.

Thus, the first systems proposed all used orthogonal space-time blockcodes.

Alamouti in “A Simple Transmit Diversity Technique for WirelessCommunications”, IEEE Journal on Selected Areas in Communications, pp.311-335, vol. 6, 1998, presented the first system using an orthogonalspace-time block code of rate 1 (where rate is defined as the ratiobetween the number N of symbols transmitted and the number L of symboltimes during which they are transmitted) for two transmit antennas.

A major drawback of Alamouti's orthogonal space-time codes is that theyare limited to two-transmit-antenna systems and that it is not possibleto extend their use directly to a system with more than two transmitantennas while keeping an unitary rate.

Tarokh and al. (“Space-time Block Codes from Orthogonal Designs”, IEEETransactions on Information Theory, 1999, 45, (5), pp. 1456-1467) thenextended the orthogonal space-time block codes to systems comprisingthree or four transmit antennas. However, the rates R=N/L obtained wereonly ½ or ¾.

One drawback of Tarokh's orthogonal space-time codes therefore is that,although they are adapted to systems implementing a greater number oftransmit antennas (three or four antennas), they have a rate of lessthan 1.

Barhumi and al. in “Pilot Tone-based Channel estimation for OFDMSystemes with Transmitter Diversity in Mobile Wireless Channels” thenproposed a channel estimation technique for multi-antenna OFDM(SISO-OFDM or MIMO-OFDM) systems relying on a classic OFDM channelestimation system, implementing an extinction of certain carriers.However, one drawback of this estimation technique in a MIMO system isthat the insertion of reference symbols generally causes major loss ofspectral efficiency whenever, for each transmit antenna, a referencesymbol is transmitted on a reference carrier at a given point in timewhile no data whatsoever is transmitted on the other carrier or carriersso as not to disturb the estimation of the transmission channel.

Other research was subsequently conducted by Fragouli and al. in“Training Based Channel Estimation for Multiple-Antenna BroadbandTransmissions” on the learning sequences that can be used for channelestimation for multi-antenna systems.

Subsequently, Stirling-Gallacher and al. (“Improving performance ofcoherent coded OFDM systems using space time transmit diversity”,Electronics Letters, Vol. 37 N, March 2001, “Practical Pilot Patternscoherent coded OFDM systems using space time transmit diversity”,European Wireless 2002 conference, 25-28 February 2002, Florence)envisaged a channel estimation technique for MIMO-OFDM systems,restricted to two-transmit-antenna systems using orthogonal space-timecodes of the Alamouti or Tarokh type.

One drawback of this estimation technique is that the number of transmitantennas of the transmission system is limited by the use of prior-artorthogonal space-time block codes.

Thus, according to the prior-art techniques, there are no complexorthogonal unit-rate codes for systems having more than two transmitantennas. This diminishes spectral efficiency.

SUMMARY

An embodiment of the invention relates to a method for transmitting adigital signal via n transmit antennas, n being strictly greater than 2,in which n vectors to be transmitted respectively by each of saidtransmit antennas are associated with a source data vector by means ofan encoding matrix M of rate equal to 1, using reference symbols knownto at least one receiver and enabling this receiver to estimate at leastthree transmission channels respectively corresponding to each of saidtransmit antennas.

According to an embodiment of the invention, said reference symbols of atransmission method of this kind undergo a mathematical transformationby said encoding matrix M before they are transmitted.

Thus, an embodiment of the invention relies on an entirely novel andinventive approach to the transmission of a digital signal, implementingan encoding matrix in a multi-antenna system with more than two transmitantennas.

More specifically, an embodiment of the invention proposes thetransmission, on the n transmit antennas, of the reference symbols ofthe encoding matrix M of rate equal to 1, a vector of reference symbolsbeing associated with the encoding matrix M by means of an encodingfunction.

Such an encoding matrix M of rate equal to 1 corresponds either to anon-orthogonal matrix or to a block orthogonal matrix, the rate beingdefined as the ratio between the number of symbols transmitted and thenumber of symbols times during which they are transmitted.

Advantageously, the reference symbols are distributed in space and intime and/or in space and in frequency.

The encoding matrix then implements a space-time encoding and/orspace-frequency encoding.

According to a first embodiment, the encoding matrix includes at leasttwo blocks, each of the blocks being orthogonal.

Preferably, each of the blocks of reference symbols is transmittedseparately, each of the blocks being transmitted on certain transmitantennas, the other transmit antennas being powered off.

Thus, the data transmitted by a first set of antennas are not disturbedby the data transmitted by another set of antennas, the other set ofantennas not transmitting on the same carriers at the same point intime.

According to another embodiment of the invention, called a thirdembodiment, the transmission method comprises a step of selectionbetween a frequency distribution and a time distribution.

In particular, this selection step may take account of thecharacteristics of a transmission channel.

According to another embodiment of the invention, called a secondembodiment, the reference symbols are transmitted on all the transmitantennas after mathematical transformation by the encoding matrix M.

Thus, the encoding matrix M is a comprehensively non-orthogonal matrix.

In particular, the encoding matrix M may be obtained by a Jafarkhanitype encoding and has the form: ${M = \left\lfloor \begin{matrix}x_{1} & x_{2} & x_{3} & x_{4} \\{- x_{2}^{*}} & x_{1}^{*} & {- x_{4}^{*}} & x_{3}^{*} \\{- x_{3}^{*}} & {- x_{4}^{*}} & x_{1}^{*} & x_{2}^{*} \\x_{4} & {- x_{3}} & {- x_{2}} & x_{1}\end{matrix} \right\rfloor},$

where x_(i) is a reference symbol and x_(i) ⁺ is a conjugate referencesymbol with i being a relative integer, 1≦i≦4.

An embodiment of the invention also relates to corresponding transmitdevice.

As indicated here above, an embodiment of the invention can thus beapplied to uplink communications, a transmit device then correspondingto a terminal (or being included in a terminal), as well as to downlinkcommunications, a transmit device corresponding, in this case, to a basestation (or being included in a base station).

An embodiment of the invention also relates to a digital signal formedby n vectors respectively transmitted by means of n transmit antennas, nbeing strictly greater than 2.

According to an embodiment of the invention, the signal comprisesencoded reference symbols, obtained after mathematical transformation ofreference symbols by an encoding matrix M of unitary rate, so as toenable the estimation, in a receiver, of at least three transmissionchannels respectively corresponding to each of the transmit antennas.

An embodiment of the invention also relates to method of estimation ofthe transmission channels in a multi-antenna system implementing ntransmit antennas, where n is strictly greater than 2, and at least onereception antenna.

According to this method, n vectors to be transmitted respectively byeach of said transmit antennas are associated with a vector of sourcedata, by means of an encoding matrix M, implementing reference symbolsknown to at least one receiver and enabling this receiver to estimate atleast three transmission channels corresponding respectively to each ofthe transmit antennas.

According to an embodiment of the invention, such an estimation methodcomprises a step of reception of a received reference vector,corresponding to a transmitted reference vector obtained by themultiplication of reference symbols by said encoding matrix M, andmodified by at least one transmission channel for each of the transmitantennas. For each of said reception antennas, the received referencevector undergoes a mathematical transformation by a decoding matrix,which is the inverse of the encoding matrix and takes account of theeffect of a transmission channel associated with the reception antenna,to give an estimation of the effects of the transmission channels on thereference symbols.

Thus, an embodiment of the invention relies on an entirely novel andinventive approach to channel estimation in a multi-antenna system withmore than two transmit antennas. It will be noted that this approach isalso novel in a system with two transmit antennas.

Indeed, the estimation of the different transmission channels isimplemented from reference symbols known to at least one receiver, avector of reference symbols being associated with an encoding matrix Mby means of an encoding function.

With the vector of reference symbols and the encoding matrix M usedbeing known, it is possible to estimate the different transmissionchannels from the inverse of the encoding matrix, corresponding to thedecoding matrix.

Thus, from reference symbols and the encoding technique used, areception device may implement techniques of decoding, filtering orequalization, and a recombination of the signals coming from the variousantennas, in order to estimate the different transmission channels.

Advantageously, the decoding matrix is an inverse matrix integrating anequalization in the sense of the MMSE (“Minimum Mean Squared Error”) orZF (“Zero Forcing”) criterion.

In particular, the criterion implemented may be the MMSE criterion. Thedecoding matrix is then formed by the elements:${\hat{h} = {\frac{M^{H}}{{M^{H}M} + \frac{I}{\gamma}}r}},$

The criterion implemented may also be the ZF criterion. The decodingmatrix is then formed by the elements:${\hat{h} = {\frac{M^{H}}{M^{H}M}r}},$

with: r being the received reference vector;

-   -   M the encoding matrix;    -   I the unitary matrix;    -   γ the signal-to-noise ratio;    -   ^(H) the conjugate transpose.

Preferably, the estimation method comprises an interpolation stepdelivering an estimation of the transmission channels for each of thepayload data from the estimation of the reference symbols.

In particular, the interpolation step is noteworthy in that itimplements a temporal interpolation and/or a frequency interpolation.

This interpolation step may belong to the group comprising:

-   -   linear interpolations;    -   Wiener interpolations.

Another embodiment of the invention relates to a reception device in amulti-antenna system implementing n transmit antennas, where n isstrictly greater than 2, and at least one reception antenna, in which nvectors to be transmitted respectively by each of said transmit antennasare associated with a vector of source data, by means of an encodingmatrix M, implementing reference symbols known to said receiver andenabling this receiver to estimate n transmission channels correspondingrespectively to each of said transmit antennas.

Such a reception device comprises means of reception of a receivedreference vector, corresponding to a transmitted reference vectorobtained by the multiplication of reference symbols by said encodingmatrix M, and modified by at least one transmission channel for each ofthe transmit antennas. For each of said reception antennas, the receivedreference vector undergoes a mathematical transformation by a decodingmatrix, which is the inverse of the encoding matrix and takes account ofthe effect of a transmission channel associated with the receptionantenna, to give an estimation of the effects of the transmissionchannels on the reference symbols.

As indicated here above, an embodiment of the invention can be appliedto uplink communications, the reception device then corresponding to abase station (or being included in a base station), as well as todownlink communications, the reception device corresponding in this caseto a terminal (or being included in a terminal).

Other features and advantages shall appear more clearly from thefollowing description of a preferred embodiment, given by way of asimple, illustrative and non-exhaustive example, and from the appendeddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B present a system of channel estimation in amulti-antenna system with four transmit antennas, with symbolsdistributed in the frequency domain (FIG. 1A) or time domain (FIG. 1B)according to a first embodiment of the invention;

FIGS. 2A and 2B present a particular distribution of the symbols of thechannel estimation system of FIGS. 1A and 1B;

FIGS. 3A and 3B illustrate a system of channel estimation in amulti-antenna system with four transmit antennas, with symbolsdistributed in the frequency domain (FIG. 3A) or time domain (FIG. 3B)according to a third embodiment of the invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The general principle of an embodiment of the invention relies on theassociation of an encoding matrix M with a vector of reference symbols,known to at least one receiver, so as to enable the estimation, in thereceiver, of the different propagation channels between more than twotransmit antennas and a reception antenna.

This encoding matrix M is either non-orthogonal or block orthogonal andhas a rate equal to 1, the rate being defined as the ratio between thenumber of symbols transmitted and the number of symbol times duringwhich they are transmitted. The symbols of the encoding matrix M arethen distributed in time and/or in frequency on each of the transmitantennas.

At reception, for each reception antenna, the received signal ismultiplied by the inverse matrix (integrating an equalizing technique asunderstood according to the MMSE or ZF criterion) of the encoding matrixM, in taking account if necessary of the noise introduced by thereceiver.

The result is a vector with n dimensions representing the n transmissionchannels between the n transmit antennas and this reception antenna.This vector with n dimensions is then used by the receiver to estimatethe transmission channels. This is done for example by repeating thisoperation periodically and performing a time and/or frequencyinterpolation between two reference symbols estimated during thisoperation. The interpolation is, for example, of a linear or Wienertype.

Referring now to FIGS. 1A and 1B, a description is given of a firstembodiment of the invention in which it is sought to estimate thetransmission channels of a multi-antenna system with four transmitantennas.

According to this first embodiment, the encoding matrix M is a blockmatrix, each of the blocks comprising n reference symbols. An Alamoutiorthogonal space-time encoding is then applied to each block of theencoding matrix M. Each of the blocks of n reference symbols is thenorthogonal.

Those skilled in the art will easily extend this teaching to the casewhere the number of antennas, in transmission and/or in reception, isgreater. It is thus possible to apply an Alamouti encoding to each ofthe blocks of a system with n=4, 6, 8, . . . transmit antennas.

According to this embodiment illustrated in FIGS. 1A and 1B, theAlamouti encoding is applied to the reference symbols used for theestimation of the channel. Then, these encoded reference symbols aretransmitted on one pair of antennas while the other pair of antennas iskept powered off.

Thus, if we consider the vector of reference symbols [x₁ x₂ x₃ x₄], theencoding matrix M which for its part is associated by means of theencoding function is: $M = \left\lfloor \begin{matrix}x_{1} & x_{2} & 0 & 0 \\{- x_{2}^{*}} & x_{1}^{*} & 0 & 0 \\0 & 0 & x_{3} & x_{4} \\0 & 0 & {- x_{4}^{*}} & x_{3}^{*}\end{matrix} \right\rfloor$

where x_(i) is a reference symbol, x_(i) ⁺ is a conjugate referencesymbol, with i as a relative integer and 1≦i≦4, and 0 signifies that nosymbol is transmitted on the concerned antenna.

Each block $\left\lfloor \begin{matrix}x_{1} & x_{2} \\{- x_{2}^{*}} & x_{1}^{*}\end{matrix} \right\rfloor\quad{and}\quad\left\lfloor \begin{matrix}x_{3} & x_{4} \\{- x_{4}^{*}} & x_{3}^{*}\end{matrix} \right\rfloor$of the encoding matrix being encoded according to an Alamouti code, wehave M·M^(H)=I, with I as the unit matrix, and ^(H) as the conjugatetranspose.

The reference symbols of the encoding matrix M are then transmittedafter space-frequency distribution (FIG. 1A) or space-time distribution(FIG. 1B) on the different transmit antennas, the space axisrepresenting the columns of the matrix M and the frequency axis (FIG.1A) or time axis (FIG. 1B) representing the rows of the matrix M.

It is clear that other space-time or space-frequency distributions ofthe symbols can be envisaged, as also a combination of the space-timeand space-frequency distributions.

In fact, each block of the encoding matrix M is transmittedindependently on its respective antennas, while the other blocks of theencoding matrix are not transmitted. In other words, each block ofreference symbols is transmitted separately, each of the blocks beingtransmitted on certain transmit antennas while the other antennas arepowered off.

Thus, FIG. 1A presents the symbols transmitted by the four antennas 11,12, 13, 14 of a multi-antenna system with four transmit antennas, thesymbols transmitted being distributed in the frequency domain (y-axis)with X_(i) being a reference symbol referenced 15, X_(i) ⁺ a conjugatereference symbol (i a relative integer and 1≦i≦4), x a data symbolreferenced 16, and 0 signifies that no symbol is transmitted.

The symbols transmitted by the four antennas 11, 12, 13, 14 aredistributed in the space-frequency domain (FIG. 1A) or space-time domain(FIG. 1B) as a function of the parameters ΔF, Δf₁, Δf₂ (FIG. 1A), ΔT,Δt₁, Δt₂ (FIG. 1B), representing the repetition patterns of thereference symbols.

The values chosen for Δf, corresponding to the spacing between tworeference carriers (in this example Δf={ΔF, Δf₁, Δf₂}), and for Δt,corresponding to the spacing between two reference symbols at knownpoints in time (in this example Δt={ΔT, Δt₁, Δt₂}), are not proper tothe proposed system but depend on the stationary state of thetransmission channel.

In general, the following are assumed:

-   -   ΔF<<B_(C), with B_(C) the coherence band of the channel;    -   ΔT<<T_(C), with T_(C) the coherence time of the channel;    -   Δ₁ verifies at best the frequency stationary state of the        channel;    -   Δt₁ verifies at best the temporal stationary state of the        channel;    -   Δf₂ and Δt₂ depend on a compromise between the loss of spectral        efficiency and the performance of the system.

FIGS. 2A and 2B also present another example of the space-timedistribution (FIG. 2A) or space-frequency distribution (FIG. 2B) of thesymbols in this first embodiment, as a function of the parameters ΔF,Δf₁, Δf₂ (FIG. 2A), ΔT, Δt₁, Δt₂ (FIG. 2B).

In this example, the values of the reference symbols are chosen so thatX₁=X₃ and X₂=X₄. The reference vector received at the level of areception antenna, modified by the transmission channel, can then bewritten in the form r=Xh+n, where h corresponds to a modeling of thetransmission channel and n is a Gaussian white noise vector.

This received reference vector can also be written in vector form:$\left\lfloor \begin{matrix}r_{1} \\r_{2} \\r_{3} \\r_{4}\end{matrix} \right\rfloor = {{\left\lfloor \begin{matrix}x_{1} & x_{2} & 0 & 0 \\{- x_{2}^{*}} & x_{1}^{*} & 0 & 0 \\0 & 0 & x_{3} & x_{4} \\0 & 0 & {- x_{4}^{*}} & x_{3}^{*}\end{matrix} \right\rfloor\left\lfloor \begin{matrix}h_{1} \\h_{2} \\h_{3} \\h_{4}\end{matrix} \right\rfloor} + \left\lfloor \begin{matrix}n_{1} \\n_{2} \\n_{3} \\n_{4}\end{matrix} \right\rfloor}$

For each of the reception antennas, it is sought to estimate thetransmission channel h by applying, to the received reference vector, amathematical transformation by a decoding matrix, corresponding to theinverse matrix integrating a technique of equalization, in the sense ofthe MMSE or ZF criterion, of the encoding matrix M.

According to the MMSE criterion, the decoding matrix is formed by theelements: ${\hat{h} = {\frac{M^{H}}{{M^{H}M} + \frac{I}{\gamma}}r}},$

with: r as said received reference vector;

-   -   M said encoding matrix;    -   I a unitary matrix;    -   γ the signal-to-noise ratio;    -   ^(H) the conjugate transpose.

According to the ZF criterion, the decoding matrix is formed by theelements: ${\hat{h} = {\frac{M^{H}}{M^{H}M}r}},$with: r as said received reference vector;

-   -   M said encoding matrix;    -   ^(H) the conjugate transpose.

These two criteria lead to identical results with a high signal-to-noiseratio.

In the case of a ZF criterion, we obtain: $\begin{bmatrix}{\hat{h}}_{1} \\{\hat{h}}_{2} \\{\hat{h}}_{3} \\{\hat{h}}_{4}\end{bmatrix} = {{{\begin{bmatrix}\frac{1}{a} & 0 & 0 & 0 \\0 & \frac{1}{a} & 0 & 0 \\0 & 0 & \frac{1}{b} & 0 \\0 & 0 & 0 & \frac{1}{b}\end{bmatrix}\begin{bmatrix}a & 0 & 0 & 0 \\0 & a & 0 & 0 \\0 & 0 & b & 0 \\0 & 0 & 0 & b\end{bmatrix}}\begin{bmatrix}h_{1} \\h_{2} \\h_{3} \\h_{4}\end{bmatrix}} + {\begin{bmatrix}\frac{1}{a} & 0 & 0 & 0 \\0 & \frac{1}{a} & 0 & 0 \\0 & 0 & \frac{1}{b} & 0 \\0 & 0 & 0 & \frac{1}{b}\end{bmatrix}M^{H}n}}$${{with}\quad a} = {{\sum\limits_{i = 1}^{2}{{x_{i}}^{2}\quad{and}\quad b}} = {\sum\limits_{i = 3}^{4}{{x_{i}}^{2}.}}}$

It is thus possible to determine the coefficients of the channel at aninstant p on a carrier k, at an instant p on a carrier k+Δf₁, . . . , asillustrated in FIG. 2A.

By applying a frequency interpolation between the two carriers k andk+Δf₁ bearing the reference symbols, the receiver can determine thecoefficients of the propagation channel at the carriers k, k+1, k+2, . .. , k+Δf₁−1, k+Δf₁.

An interpolation can also be made in the time domain, in consideringthat the reference symbols are transmitted at the instant p on thecarrier k, at the instant p+Δt on the same carrier k, . . . . Thereceiver can then determine the coefficients of the propagation channelat the instants p, p+1, p+2, . . . , p+Δt−1, p+Δt and so on and soforth.

The receiver can therefore perform a time interpolation and/or afrequency interpolation. This interpolation step implements aninterpolation technique well known to those skilled in the art, such asfor example a linear type interpolation or a Wiener interpolation.

Since the other pair of antennas does not transmit on the same carriersand at the same instants, as illustrated in FIGS. 1A, 1B, 2A and 2B, thesignal transmitted by the first pair of antennas is not disturbed.

Each pair of antennas then alternately transmits the reference symbolsdistributed on its antennas, so as to estimate all the transmissionchannels of the multi-antenna system.

According to an embodiment of the invention, it is thus possible toapply orthogonal space-time codes to systems having a greater number oftransmit antennas, by means of an encoding matrix M preserving a rateequal to 1. It is thus possible to apply an Alamouti code with a rateequal to 1 to systems having 4, 6, 8, . . . , transmit antennas (whereasin the prior art, the number of transmit antennas of the transmissionsystem is limited owing to the use of orthogonal space-time codes).

However, although this channel estimation technique performs better interms of estimation, since the other groups of antennas are powered offwhen one group of antennas is transmitting, this technique isaccompanied by a loss of spectral efficiency and does not benefit fromthe total power of the antennas since certain carriers convey noinformation at defined instants.

A second embodiment of the invention is then presented, wherein thereference symbols are transmitted on all the transmit antennas aftermathematical transformation by the encoding matrix M, the encodingmatrix M being non-orthogonal.

According to the second embodiment, a Jafarkhani type non-orthogonalspace-time encoding, as presented in “A Quasi-Orthogonal Space-TimeBlock Code” (IEEE Transactions on Communications, Vol. 49, N^(o)1, 2001,pp. 1-4), is applied to the reference symbols used for the estimation ofthe channel.

This encoding is used especially to transmit signals showing lowinterference.

Thus, if we consider the vector of reference symbols [x₁ x₂ x₃ x₄], theencoding matrix M associated with it by means of the encoding functionis: ${M = \left\lfloor \begin{matrix}x_{1} & x_{2} & x_{3} & x_{4} \\{- x_{2}^{*}} & x_{1}^{*} & {- x_{4}^{*}} & x_{3}^{*} \\{- x_{3}^{*}} & {- x_{4}^{*}} & x_{1}^{*} & x_{2}^{*} \\x_{4} & {- x_{3}} & {- x_{2}} & x_{1}\end{matrix} \right\rfloor},$

where x_(i) is a reference symbol, x₁ ⁺ is a conjugate reference symbolwith i as a relative integer, 1≦i≦4.

All the reference symbols of the encoding matrix M are then transmittedafter space/frequency distribution on all the transmit antennas, thespatial axis representing the columns of the matrix M and the frequencyor time axis representing the rows of the matrix M.

As described here above, the reference vector received at a receptionantenna, modified by the transmission channel, can be written in theform r=Xh+n, where h corresponds to a modeling of the transmissionchannel and n is a Gaussian white noise vector.

This received reference vector can also be written in vector form:$\begin{bmatrix}r_{1} \\r_{2} \\r_{3} \\r_{4}\end{bmatrix} = {{\begin{bmatrix}x_{1} & x_{2} & x_{3} & x_{4} \\{- x_{2}^{*}} & x_{1}^{*} & {- x_{4}^{*}} & x_{3}^{*} \\{- x_{3}^{*}} & {- x_{4}^{*}} & x_{1}^{*} & x_{2}^{*} \\x_{4} & {- x_{3}} & {- x_{2}} & x_{1}\end{bmatrix} \cdot \begin{bmatrix}h_{1} \\h_{2} \\h_{3} \\h_{4}\end{bmatrix}} + {\begin{bmatrix}n_{1} \\n_{2} \\n_{3} \\n_{4}\end{bmatrix}.}}$

Once again, for each of the reception antennas, it is sought to estimatethe transmission channel h by the application, to the received referencevector, of a mathematical transformation by a decoding matrix,corresponding to the inverse matrix integrating a technique ofequalization in the sense of the MMSE or ZF criterion of the encodingmatrix M.

In the case of an MMSE criterion, we get: $\begin{bmatrix}{\hat{h}}_{1} \\{\hat{h}}_{2} \\{\hat{h}}_{3} \\{\hat{h}}_{4}\end{bmatrix} = {{{\begin{bmatrix}{a + \frac{1}{\gamma}} & 0 & 0 & b \\0 & {a + \frac{1}{\gamma}} & {- b} & 0 \\0 & {- b} & {a + \frac{1}{\gamma}} & 0 \\b & 0 & 0 & {a + \frac{1}{\gamma}}\end{bmatrix}^{- 1}\begin{bmatrix}a & 0 & 0 & b \\0 & a & {- b} & 0 \\0 & {- b} & a & 0 \\b & 0 & 0 & a\end{bmatrix}} \cdot \begin{bmatrix}h_{1} \\h_{2} \\h_{3} \\h_{4}\end{bmatrix}} + {\begin{bmatrix}{a + \frac{1}{\gamma}} & 0 & 0 & b \\0 & {a + \frac{1}{\gamma}} & {- b} & 0 \\0 & {- b} & {a + \frac{1}{\gamma}} & 0 \\b & 0 & 0 & {a + \frac{1}{\gamma}}\end{bmatrix}^{- 1}{M^{H} \cdot n}}}$${{with}\quad a} = {{\sum\limits_{i = 1}^{4}{{x_{i}}^{2}{and}\quad b}} = {2{{{Re}\left( {{x_{1}x_{4}^{*}} - {x_{2}x_{3}^{*}}} \right)}.}}}$

This operation is reiterated identically for each reception antenna,whatever the number of antennas. It is thus possible to determine thecoefficients h_(c) of a transmission channel at a frequency c or at adefined instant c, and all that remains to be done is to apply a timeand/or frequency interpolation at the receiver between the estimates ofh_(c) and h_(c+k) (with k=Δf should c be a frequency and k=Δt should cbe an instant) in order to assess the missing values. Thus,comprehensive knowledge is obtained of all the values of a transmissionchannel for each of the antennas, thus making it possible to equalizethe reception signal conventionally.

According to this second embodiment, it is possible to extend thisestimation technique to systems having more than two transmit antennas.

Thus, if we consider the vector of reference symbols [x₁ x₂ . . .x_(n)], the encoding matrix M that is associated with it by means of theencoding function is: ${M = \left\lfloor \begin{matrix}x_{1} & \ldots & x_{n} \\\vdots & ⋰ & \vdots \\x_{N - n + 1} & \ldots & x_{N}\end{matrix} \right\rfloor},$

where x_(i) is a reference symbol, with i as a relative integer 1≦i≦N,and N=n².

This is a full-ranking matrix so that it can be inverted during theestimation of the different channels.

As described here above, the reference symbols of the encoding matrix Mare transmitted after space/frequency distribution on all the transmitantennas, and the reference vector received at a reception antenna,modified by the transmission channel, can be written in the form:$\left\lfloor \begin{matrix}r_{1} \\r_{2} \\r_{3} \\r_{4}\end{matrix} \right\rfloor = {{\left\lfloor \begin{matrix}x_{1} & x_{2} & x_{3} & x_{4} \\x_{5} & x_{6} & x_{7} & x_{8} \\x_{9} & x_{10} & x_{11} & x_{12} \\x_{13} & x_{14} & x_{15} & x_{16}\end{matrix} \right\rfloor \cdot \left\lfloor \begin{matrix}h_{1} \\h_{2} \\h_{3} \\h_{4}\end{matrix} \right\rfloor} + {\left\lfloor \begin{matrix}n_{1} \\n_{2} \\n_{3} \\n_{4}\end{matrix} \right\rfloor.}}$

Once again, for each of the reception antennas, the received referencevector receives the application of a mathematical transformation bymeans of a decoding matrix, corresponding to the inverse matrixintegrating a technique of equalization in the sense of the MMSE or ZFcriterion of the encoding matrix M in order to estimate the transmissionchannel h.

If an equalization technique in the sense of the MMSE criterion is used,we get: ${\begin{bmatrix}{\hat{h}}_{1} \\{\hat{h}}_{2} \\{\hat{h}}_{3} \\{\hat{h}}_{4}\end{bmatrix} = {{\left\lbrack {{M^{H}M} + \frac{I}{\gamma}} \right\rbrack^{- 1}{M^{H} \cdot \begin{bmatrix}h_{1} \\h_{2} \\h_{3} \\h_{4}\end{bmatrix}}} + {\left\lbrack {{M^{H}M} + \frac{I}{\gamma}} \right\rbrack^{- 1}M^{H}n}}},$with γ the signal-to-noise ratio.

As described here above, it is then possible to determine the missingcoefficients of the transmission channel by applying a time or frequencyinterpolation (or both) to the receiver in using a classic technique ofinterpolation.

Referring to FIGS. 3A and 3B, we now present a third embodiment of theinvention that can be applied more particularly to MIMO typemulti-antennas systems.

In this third embodiment, a flexible principle is proposed for theapplication of either a space-time encoding or a space-frequencyencoding, depending on the characteristics of the transmission channel.

Thus, FIG. 3A illustrates the transmission of four reference symbols andtheir conjugates temporally spaced out, in a multi-antenna system withfour transmit antennas with X_(i) being a reference symbol referenced15, X_(i) ⁺ a conjugate reference symbol (i a relative integer and1≦i≦4), x a data symbol, referenced 16.

FIG. 3B illustrates the transmission of four reference symbols and theirconjugates spaced out frequentially, in a multi-antenna system with fourtransmit antennas.

In this third embodiment, the reference symbols, once encoded by meansof the encoding matrix M, are distributed along the time axis orfrequency axis according to the properties of the propagation channel.

It is then possible to switch from a space-time encoding to aspace-frequency encoding.

It may be recalled that the values chosen for Δf (spacing between tworeference carriers) and Δt (spacing between two reference symbols atknown instants) are not proper to the proposed system but dependrespectively on the band and time of coherence of the transmissionchannel.

As a general rule, the distribution in the time domain is applied ratherin the case of the channel that varies temporally while the frequencydistribution is applied more to a channel that varies frequentially.

Thus, with a priori knowledge of the channel or having computed thevalues of the coherence band or the coherence time of the channel, it ispossible to switch between the two structures of insertion of referencesymbols described here above.

Those skilled in the art will easily extend the teaching of these threeembodiments to systems having a greater number of antennas, as well asto systems having a space-time distribution and/or a space-frequencydistribution different from those proposed in FIGS. 1A, 1B, 2A, 2B, 3Aand 3B.

Thus, according to an embodiment of the invention, the differenttransmit antennas transmit, on a same carrier and at a same instant, asignal characterized by a space-time encoding and/or space-frequencyencoding, thus limiting the loss of spectral efficiency.

This signal therefore intrinsically comprises the characteristics of anembodiment of the invention.

Finally, a receiver may estimate each of the transmission channelsbetween the different transmit and reception antennas on the basis ofthis specific encoding and of the appropriate processing described hereabove. The particular channel estimation technique proposed according toan embodiment of the invention may therefore be applied in the case of asystem having two transmit antennas.

An embodiment of the invention provides a technique for the estimationof transmission channels in a multi-antenna system implementing morethan two transmit antennas.

An embodiment of the invention proposes a technique of this kind that ismore efficient and performs better than the prior-art techniques whilehaving lower complexity.

An embodiment of the invention provides a technique of transmission of asignal comprising reference symbols implementing a space-time encodingand/or space-frequency encoding matrix. In particular, an embodiment ofthe invention provides a unit-rate encoding matrix.

An embodiment provides a technique of this kind that is adapted to MISOor MIMO type multi-antenna systems for single-carrier or multi-carriertype modulations combined with the different techniques of multipleaccess, namely CDMA (Code Division Multiplex Access), FDMA (frequencydivision multiple access) or TDMA (time division multiple access).

An embodiment of the invention proposes a technique of this kind thatcan be used to augment the spatial diversity of the systems while at thesame time reducing interference between the different transmissionchannels to the minimum and limiting the loss of spectral efficiency.

In other words, an embodiment of the invention provides a technique ofthis kind that can be implemented in a practical and low-cost manner ina system implementing a large number of antennas.

Although the present disclosure has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the disclosure.

1. Method of transmission comprising: transmitting a digital signal vian transmit antennas, n being strictly greater than 2, in which n vectorsto be transmitted respectively by each of said transmit antennas areassociated with a source data vector by an encoding matrix M of rateequal to 1, using reference symbols known to at least one receiver andenabling this receiver to estimate n transmission channels respectivelycorresponding to each of said transmit antennas; and mathematicallytransforming said reference symbols by said encoding matrix M beforethey are transmitted.
 2. Method of transmission according to claim 1,wherein said reference symbols are distributed in space and in time. 3.Method of transmission according to claim 1, wherein said referencesymbols are distributed in space and in frequency.
 4. Method oftransmission according to claim 1, wherein said encoding matrix includesat least two blocks, each of said blocks being orthogonal.
 5. Method oftransmission according to claim 4, wherein each of said blocks ofreference symbols is transmitted separately, each of said blocks beingtransmitted on certain of said transmit antennas, the other transmitantennas being powered off.
 6. Method of transmission according to claim1, and further comprising a step of selection between a frequencydistribution and a time distribution.
 7. Method of transmissionaccording to claim 6, wherein said selection step takes account of thecharacteristics of a transmission channel.
 8. Method of transmissionaccording to claim 1, wherein said reference symbols are transmitted onall the transmit antennas after mathematical transformation by saidencoding matrix M.
 9. Method of transmission according to claim 8,wherein said encoding matrix is a matrix obtained by a Jafarkhani typeencoding and has the form: ${M = \left\lfloor \begin{matrix}x_{1} & x_{2} & x_{3} & x_{4} \\{- x_{2}^{*}} & x_{1}^{*} & {- x_{4}^{*}} & x_{3}^{*} \\{- x_{3}^{*}} & {- x_{4}^{*}} & x_{1}^{*} & x_{2}^{*} \\x_{4} & {- x_{3}} & {- x_{2}} & x_{1}\end{matrix} \right\rfloor},$ where x_(i) is a reference symbol andx_(i) ⁺ is a conjugate reference symbol with i being a relative integer,1≦i≦4.
 10. Digital signal formed by n vectors respectively transmittedby n transmit antennas, n being strictly greater than 2, wherein thesignal comprises encoded reference symbols, obtained after mathematicaltransformation of reference symbols by an encoding matrix M of rateequal to 1, so as to enable the estimation, in a receiver, of said ntransmit channels respectively corresponding to each of said transmitantennas.
 11. Method of estimation of the transmit channels in amulti-antenna system implementing n transmit antennas, where n isstrictly greater than 2, and at least one reception antenna, accordingto which n vectors to be transmitted respectively by each of saidtransmit antennas are associated with a vector of source data, by anencoding matrix M, implementing reference symbols known to at least onereceiver and enabling this receiver to estimate n transmit channelscorresponding respectively to each of said transmit antennas, whereinthe method comprises: receiving a received reference vector,corresponding to a transmitted reference vector obtained by themultiplication of reference symbols by said encoding matrix M, andmodified by at least one transmission channel for each of said transmitantennas; and for each of said reception antennas, mathematicallytransforming said received reference vector by a decoding matrix, whichis the inverse of said encoding matrix and takes account of the effectof a transmission channel associated with said reception antenna, togive an estimation of the effects of said transmit channels on saidreference symbols.
 12. Method of estimation according to claim 11,wherein said decoding matrix is an inverse matrix integrating anequalization in the sense of the MMSE or ZF criterion.
 13. Method ofestimation according to claim 12, wherein said criterion implemented isthe MMSE criterion and said decoding matrix is formed by the elements:${\hat{h} = {\frac{M^{H}}{{M^{H}M} + \frac{I}{\gamma}}r}},$ with: rbeing the received reference vector; M the encoding matrix; I a unitarymatrix; γ the signal-to-noise ratio; ^(H) the conjugate transpose. 14.Method of estimation according to claim 12, wherein said criterionimplemented is the ZF criterion and in that said decoding matrix isformed by the elements: ${\hat{h} = {\frac{M^{H}}{M^{H}M}r}},$ with: rbeing said received reference vector; M said encoding matrix; ^(H) theconjugate transpose.
 15. Method of estimation according to claim 11, andfurther comprising an interpolation step delivering an estimation ofsaid transmission channels, for each of the source data, from theestimation of the reference symbols.
 16. Method of estimation accordingto claim 15, wherein said interpolation step implements a temporalinterpolation and/or a frequency interpolation.
 17. Method of estimationaccording to claim 15 wherein said interpolation step belongs to thegroup comprising: linear interpolations; Wiener interpolations. 18.Device for transmitting a digital signal feeding n transmit antennas, nbeing strictly greater than 2, comprising: an encoding matrix M of rateequal to 1, which associates n vectors to be transmitted respectively byeach of said transmit antennas with a source data vector, usingreference symbols known to at least one receiver and enabling thisreceiver to estimate n transmission channels respectively correspondingto each of said transmit antennas, and an encoder, which applies amathematical conversion to said reference symbols by said encodingmatrix M.
 19. Device for transmitting according to claim 18, wherein thedevice forms or is integrated into at least one of the installationsbelonging to the group comprising: a base station; a terminal. 20.Reception device in a multi-antenna system, implementing n transmitantennas, where n is strictly greater than 2, and at least one receptionantenna in which n vectors to be transmitted respectively by each ofsaid transmit antennas are associated with a vector of source data, anencoding matrix M, implementing reference symbols known to said receiverand enabling this receiver to estimate n transmit channels correspondingrespectively to each of said transmit antennas, wherein the receptiondevice comprises: a receiver, which receives a received referencevector, corresponding to a transmitted reference vector obtained by themultiplication of reference symbols by said encoding matrix M, andmodified by at least one transmission channel for each of said transmitantennas, and a decoder which, for each of said reception antennas,mathematically transforms said received reference vector by a decodingmatrix, which is the inverse of the encoding matrix and takes account ofthe effect of a transmission channel associated with said receptionantenna, to give an estimation of the effects of said transmit channelson said reference symbols.
 21. Reception device according to claim 20,wherein the device forms or is integrated into at least one of theinstallations belonging to the group comprising: a base station; aterminal.